Field of the Invention
The present invention relates to hydrotreating processes to efficiently reduce the sulfur content of hydrocarbons.
Description of Related Art
The discharge into the atmosphere of sulfur compounds during processing and end-use of the petroleum products derived from sulfur-containing sour crude oil poses health and environmental problems. Stringent reduced-sulfur specifications applicable to transportation and other fuel products have impacted the refining industry, and it is necessary for refiners to make capital investments to greatly reduce the sulfur content in gas oils to 10 parts per million by weight (ppmw) or less. In the industrialized nations such as the United States, Japan and the countries of the European Union, refineries have already been required to produce environmentally clean transportation fuels. For instance, in 2007 the United States Environmental Protection Agency required the sulfur content of highway diesel fuel to be reduced 97%, from 500 ppmw (low sulfur diesel) to 15 ppmw (ultra-low sulfur diesel). The European Union has enacted even more stringent standards, requiring diesel and gasoline fuels sold in 2009 to contain less than 10 ppmw of sulfur. Other countries are following in the footsteps of the United States and the European Union and are moving forward with regulations that will require refineries to produce transportation fuels with ultra-low sulfur levels.
To keep pace with recent trends toward production of ultra-low sulfur fuels, refiners must choose among the processes or crude oils that provide flexibility that ensures future specifications are met with minimum additional capital investment, in many instances by utilizing existing equipment. Conventional technologies such as hydrocracking and two-stage hydrotreating offer solutions to refiners for the production of clean transportation fuels. These technologies are available and can be applied as new grassroots production facilities are constructed. However, many existing hydroprocessing facilities, such as those using relatively low pressure hydrotreaters, represent a substantial prior investment and were constructed before these more stringent sulfur reduction requirements were enacted. It is very difficult to upgrade existing hydrotreating reactors in these facilities because of the comparatively more severe operational requirements (i.e., higher temperature and pressure) to obtain clean fuel production. Available retrofitting options for refiners include elevation of the hydrogen partial pressure by increasing the recycle gas quality, utilization of more active catalyst compositions, installation of improved reactor components to enhance liquid-solid contact, the increase of reactor volume, and the increase of the feedstock quality.
There are many hydrotreating units installed worldwide producing transportation fuels containing 500-3000 ppmw sulfur. These units were designed for, and are being operated at, relatively mild conditions (i.e., low hydrogen partial pressures of 30 kilograms per square centimeter for straight run gas oils boiling in the range of from 180° C. to 370° C.).
With the increasing prevalence of more stringent environmental sulfur specifications in transportation fuels mentioned above, the maximum allowable sulfur levels are being reduced to no greater than 15 ppmw, and in some cases no greater than 10 ppmw. This ultra-low level of sulfur in the end product typically requires either construction of new high pressure hydrotreating units, or a substantial retrofitting of existing facilities, e.g., by incorporating gas purification systems, reengineering the internal configuration and components of reactors, and/or deployment of more active catalyst compositions.
Sulfur-containing compounds that are typically present in hydrocarbon fuels include aliphatic molecules such as sulfides, disulfides and mercaptans as well as aromatic molecules such as thiophene, benzothiophene and its long chain alkylated derivatives, and dibenzothiophene and its alkyl derivatives such as 4,6-dimethyl-dibenzothiophene. Aromatic sulfur-containing molecules have a higher boiling point than aliphatic sulfur-containing molecules, and are consequently more abundant in higher boiling fractions.
In addition, certain fractions of gas oils possess different properties. Table 1 illustrates the properties of light and heavy gas oils derived from Arabian Light crude oil:
TABLE 1Feedstock NameLightHeavyAPI Gravity°37.530.5CarbonW %85.9985.89HydrogenW %13.0712.62SulfurW %0.951.65Nitrogenppmw42225ASTM D86 DistillationIBP/5 V %° C.189/228147/24410/30 V %° C.232/258276/32150/70 V %° C.276/296349/37385/90 V %° C.319/330392/39895 V %° C.347Sulfur SpeciationOrganosulfur Compoundsppmw45913923Boiling Less than 310° C.Dibenzothiophenesppmw10412256C1- Dibenzothiophenesppmw14412239C2- Dibenzothiophenesppmw13252712C3- Dibenzothiophenesppmw11045370
As set forth above in Table 1, the light and heavy gas oil fractions have ASTM D85/90 V % point of 319° C. and 392° C., respectively. Further, the light gas oil fraction contains less sulfur and nitrogen than the heavy gas oil fraction (0.95 weight % or W % sulfur as compared to 1.65 W % sulfur and 42 ppmw nitrogen as compared to 225 ppmw nitrogen).
Advanced analytical techniques such as multi-dimensional gas chromatography (Hua R., Li Y., Liu W., Zheng J., Wei H., Wang J., LU X., Lu X., Kong H., Xu G., Journal of Chromatography A, 1019 (2003) 101-109) with a sulfur chemiluminescence detector have shown that the middle distillate cut boiling in the range of from 170° C. to 400° C. contains sulfur species including thiols, sulfides, disulfides, thiophenes, benzothiophenes, dibenzothiophenes, and benzonaphthothiophenes, with and without alkyl substituents.
The sulfur speciation and content of light and heavy gas oils are conventionally analyzed by two methods. In the first method, sulfur species are categorized based on structural groups. The structural groups include one group having sulfur-containing compounds boiling at less than 310° C., including dibenzothiophenes and its alkylated isomers, and another group including 1, 2 and 3 methyl-substituted dibenzothiophenes, denoted as C1, C2 and C3, respectively. Based on this method, the heavy gas oil fraction contains more alkylated di-benzothiophene molecules than the light gas oils.
In the second method of analyzing sulfur content of light and heavy gas oils, and referring to FIG. 1, the cumulative sulfur concentrations are plotted against the boiling points of the sulfur-containing compounds to observe concentration variations and trends. Note that the boiling points depicted are those of detected sulfur-containing compounds, rather than the boiling point of the total hydrocarbon mixture. The boiling point of several refractory sulfur-containing compounds consisting of dibenzothiophenes, 4-methyldibenzothiophenes and 4,6-dimethyl-dibenzothiophenes are also shown in FIG. 1 for convenience. The cumulative sulfur specification curves show that the heavy gas oil fraction contains a higher proportion of heavier sulfur-containing compounds and lower proportion of lighter sulfur-containing compounds as compared to the light gas oil fraction. For example, it is found that 5370 ppmw of C3-dibenzothiophene, and bulkier molecules such as benzonaphthothiophenes, are present in the heavy gas oil fraction, compared to 1104 ppmw in the light gas oil fraction. In contrast, the light gas oil fraction contains a higher content of light sulfur-containing compounds compared to heavy gas oil. Light sulfur-containing compounds are structurally less bulky than dibenzothiophenes and boil at less than 310° C. Also, twice as much C1 and C2 alkyl-substituted dibenzothiophenes exist in the heavy gas oil fraction as compared to the light gas oil fraction.
Aliphatic sulfur-containing compounds are more easily desulfurized (labile) using mild hydrodesulfurization methods. However, certain highly branched aromatic molecules can sterically hinder the sulfur atom removal and are moderately more difficult to desulfurize (refractory) using mild hydrodesulfurization methods.
Among the sulfur-containing aromatic compounds, thiophenes and benzothiophenes are relatively easy to hydrodesulfurize. The addition of alkyl groups to the ring compounds increases the difficulty of hydrodesulfurization. Dibenzothiophenes resulting from addition of another ring to the benzothiophene family are even more difficult to desulfurize, and the difficulty varies greatly according to their alkyl substitution, with di-beta substitution being the most difficult to desulfurize, thus justifying their “refractory” appellation. These beta substituents hinder exposure of the heteroatom to the active site on the catalyst.
The economical removal of refractory sulfur-containing compounds is therefore exceedingly difficult to achieve, and accordingly removal of sulfur-containing compounds in hydrocarbon fuels to an ultra-low sulfur level is very costly by current hydrotreating techniques. When previous regulations permitted sulfur levels up to 500 ppmw, there was little need or incentive to desulfurize beyond the capabilities of conventional hydrodesulfurization, and hence the refractory sulfur-containing compounds were not targeted. However, in order to meet the more stringent sulfur specifications, these refractory sulfur-containing compounds must be substantially removed from hydrocarbon fuels streams.
Relative reactivities of sulfur-containing compounds based on their first order reaction rates at 250° C. and 300° C. and 40.7 Kg/cm2 hydrogen partial pressure over Ni—Mo/alumina catalyst, and activation energies, are given in Table 2 (Steiner P. and Blekkan E. A., Fuel Processing Technology 79 (2002) 1-12).
TABLE 24-methyl-dibenzo-4,6-dimethyl-NameDibenzothiophenethiophenedibenzo-thiopheneStructure Reactivity k@250, s−157.710.41.0Reactivity k@300, s−17.32.51.0Activation Energy28.736.153.0Ea, Kcal/mol
As is apparent from Table 2, dibenzothiophene is 57 times more reactive than the refractory 4, 6-dimethyldibenzothiphene at 250° C. The relative reactivity decreases with increasing operating severity. With a 50° C. temperature increase, the relative reactivity of di-benzothiophene compared to 4,6-dimethyl-dibenzothiophene decreases to 7.3 from 57.7.
Studies have been conducted related to increasing the relative reactivity of sterically hindered sulfur-containing hydrocarbons. In particular, isomerization of 4,6-dimethyl-dibenzothiophene into methyl-migrated isomers and tri- or tetramethyl-dibenzothiophenes was studied. See Isoda et al., “Hydrodesulfurization Pathway of 4,6-Dimethyldibenzothiophene through Isomerization over Y-Zeolite Containing CoMo/Al2O3 Catalyst”, Energy & Fuels, 1996, 10, 1078-1082, and Isoda et al., “Changes in Desulfurization Reactivity of 4,6-Dimethyldibenzothiophene by Skeletal Isomerization Using a Ni-Supported Y-Type Zeolite,” Energy & Fuels, 2000, 14, 585-590.
McConnachie, et al. U.S. Pat. No. 7,731,838 describes a process in which a diesel boiling range feedstream which includes nitrogen-containing compounds and hindered dibenzothiophenes is subjected to removal of nitrogen-containing compounds by contacting the feedstream with a sulfuric acid solution, contacting the reduced nitrogen stream with a solid acid catalyst to isomerize certain sulfur-containing molecules; and hydrotreating the isomerized effluent. However, the described process in McConnachie, et al. prefers that the entire feed is both subjected to the removal of nitrogen-containing compounds by contacting the feedstream with a sulfuric acid solution, and contacted with the costly and sensitive (i.e., easily poisoned) solid acid isomerization catalyst. This requires that the nitrogen-removal apparatus have a capacity suitable for the entire feedstream, and impart excessive demand on this isomerization catalyst and its reactor capacity.
McVicker, et al. U.S. Pat. No. 5,897,768 teaches a desulfurization process in which an entire feedstream is hydrotreated using conventional catalysts. The partially hydrotreated effluent is fractionated, whereby sterically hindered sulfur-containing hydrocarbons are removed with the bottoms stream. The bottoms stream is passed to a reactor containing isomerization catalyst. The effluent from the reactor containing isomerization catalyst is returned to the hydrotreating reactor. However, in McVicker, et al., the entire initial feed is passed through the hydrotreating reactor, including refractory sulfur-containing hydrocarbons which are likely not desulfurized in this initial pass-through, thus decreasing the overall process efficiency.
Therefore, a need exists for improved process and apparatus for desulfurization of hydrocarbon fuels to an ultra-low sulfur level.
Accordingly, it is an object of the present invention to desulfurize a hydrocarbon fuel stream containing different classes of sulfur-containing compounds having different reactivities.